miles per gallon

[GNX261]

Good Morning. I recently purchased a 1987 Buick Grand National and have a question or two.

1. At what speed or RPM does the car shift into overdrive. My daily commute seldom gets me there so I am driving in 3rd a lot which is equivalent to about 1880-2000 rpms and 35 to 40 miles per hour. Could this be the cause of my ability to only get about 10 miles per gallon? The injectors and fuel system are stock and I just replaced the plugs, wires, and oxygen sensor.

2. I have an external wastegate and am wondering how I adjust it and why I would want to.

3. The previous owner adjusted the boost to a maximum of 13. How can I increase it the stock 15? Also, how much boost can I do without modifying the fuel system?

Thanks, folks.

 

[scottyb]

1. IIRC generally stock and cruising, 4th gear came ~ 40-45 mph. I rarely see 4th gear also, and I get ~ 14 mpg.

2. dunno

3. don't go screwing around with it until you have confirmed upgraded fuel pump, and you have a scan tool.

A scan tool also may help you determine your poor MPG.

 

[VandaLL]

here's some good tech articles on these subjects (boost and wastegate) to get started on.

http://gnttype.org/techarea/turbo/turbopage.html

 

[2QUIK6]

With the stock chip, OD comes on very soon...can't remember exactly but I think it was around 30-35mph. Most aftermarket chip raise that to around 40-45mph. Bad gas mileage could be from that or more likely a lazy O2 sensor....as mentioned, get a scan tool tool before ever going WOT....the scan tool will also tell you about the O2 or not.

 

[bruce]

I am driving in 3rd a lot which is equivalent to about 1880-2000 rpms and 35 to 40 miles per hour. Could this be the cause of my ability to only get about 10 miles per gallon?

Yes.

Try using drive, and see if that improves it. At 35, I doubt you'll ever get any *good* mileage. FWIW, the best mileage I ever got was cruising at a little over 70 MPH, in 90d heat, in 90% humidity.

 

[VadersV6]

Do a search on "mileage", and you'll find just about every possibility. I got 8mpg highway when I first got mine. Now I get about 16, but its only that bad because of my high stall, non lockup torque converter.
I havent replaced my fuel pump yet, and I run 20-22lbs of boost. But, I have a scan tool for monitoring knock, Ive learned how to tune and I dont stay at wide open throttle for more than 4 seconds or so. Keep the boost where its at until you buy a scan tool, and learn all the ins and outs of this car. Knowledge will save it! Learn everything you can, from a fan relay all the way to the code in the computer. All the technical articles on gnttype.org are a great place to start. Read through the forums every day. Things will go wrong out of nowhere, and being familiar with the car will keep it on the road.